The present invention relates to a rust proof film-forming method for treating the surface of metal materials to thus effectively keep the same from rusting and a liquid rust proof film-forming composition for use in the method.
There have been used a solution containing hexavalent chromium in most of the conventionally proposed techniques for treating the surface of metals to thus effectively keeping the metal surface from rusting. The hexavalent chromium is a quite efficient rust proofing agent, but is highly toxic and adversely affects environment and human health. For this reason, there have been proposed a variety of methods for preventing rusting without using hexavalent chromium.
For instance, Japanese Un-Examined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 52-92836 discloses a method for forming a conversion film on the surface of zinc and zinc alloys by treating the surface with an aqueous solution comprising titanium ions and at least one member selected from the group consisting of phosphoric acid, phytic acid, tannic acid and hydrogen peroxide and J. P. KOKAI No. Sho 57-145987 discloses a method for forming a conversion film on the surface of aluminum and aluminum alloys by treating the same with an aqueous solution comprising, as principal components, a silicate and a zinc compound. However, these methods do not necessarily impart sufficient corrosion resistance practically acceptable to the metal surface and cannot supersede the treating methods using hexavalent chromium.